1. Field of the Invention
This invention relates to an inking procedure and mechanism designed for typographic, offset, flexographic, lithographic and other printing presses.
2. Description of the Prior Art
The inking mechanisms used to equip nearly all printing presses are analogous to one another and make possible only the use of one ink per printing.
They are constructed and operate as follows:
A duct serves as reservoir and ink source, and flow is obtained from it through the interplay between the deformation in a flexible steel blade and a metal cylinder, called the ink drum, against which the blade presses. The ink comes into play between the blade and the ink drum and forms as its surface a film of varying thickness depending on the pressure applied to the back of the blade by control screws.
A set of flexible rollers, alternated with metal cylinders, transfers and modifies the ink film to make it ready for inking. Depending on the manufacturer and on the density and quality of inking desired, the number, diameter and arrangement of the rollers and cylinders may vary significantly.
According to this principle of inking, it is possible only approximately to control the thickness of the ink film in zones about 20 mm to 40 mm in width. The advance of the ink drum is generally variable and adjustable.
The film emerging from the duct is relatively thick and not suited to high quality printing; it is improved by each distribution roller which thinly spreads, laminates, mixes and homogenizes it. The distributing cylinders take part in and complete this action; they are called "distributors" because of their axial and rotary movement. Area by area, they even out the thickness of the film and prevent annular ridges which might result from ink surface tension.
The film inking the printing portions of presses must be perfectly even and of constant thickness in a given zone. The acceptable tolerance is of the order of 2 to 3 microns above or below the target thickness.
Depending on the holder, the type of printing and various other factors, the ink forming the film must have a particular rheology, which is determined by its ingredients, possible additives, and the mechanical action of the inking device. To a large degree, the quality of the inking determines the quality of the ultimate print.
Conventional inking mechanisms in general make it possible to meet such requirements. In addition to their bulk and clumsiness, however, they present a number of drawbacks:
imprecise control of ink film thickness in narrow zones, making the juxtaposition of flat tint printing with small characters quite tricky;
unnecessarily excessive ink consumption when the surface area to be printed is negligible in comparison with the overall printing surface of the machine. This consumption becomes very high when there are a number of short printing runs to be made in succession using different colors;
when printing is first started up, the ink balance is obtained only after a certain number of copies have been printed, resulting in paper waste.
Moreover, the basic principle of this type of inking makes it unsuitable for handling different colors of ink in a single printing. Indeed, the so-called distributor cylinders, those which move axially and in rotation, rapidly mix the inks together, and the duct does not allow for the release and control of narrow flows of inks of different colors.
In an effort to overcome this limitation, several patents have been filed. They all retain the same printing principle and, by means of more or less different methods, suggest the creation of zones of discontinuity at the junctions between the different-colored inks by using circumferential grooves in the axially-moving rollers or the rollers working in tandem, or by scraping up the residual ink which remains after the mixing action of the axially-moving rollers. These methods are but palliatives, and are ill adapted to successive runs which may differ greatly from one to the next and often are quite short in numerical terms. Quite apart from discussion of the duct, the sole fact of having to make special "distributor rollers" for each different press run gives rise to additional work for the printer, work which in the vast majority of cases is not justified by the time savings achieved and, in practice, eliminates the advantages of this method.
Indeed, a printer who desires a multicolored printing is required to treat each color in succession, one after another, which does not require as many passes through the press as it does colors except in three- or four-colored prints using plates where the overlaying of the three primary colors theoretically makes it possible to achieve any tint. However, this type of printing is reserved for specific types of prints and still requires 3 or 4 press runs.
This de facto situation makes it difficult to produce multicolor prints at a low cost.
French Pat. Nos. 1.275.206, 1.341.700, and 2.194.576 have proposed solutions aimed at simplifying conventional inking devices by eliminating almost all elements of the sequence of distribution rollers and cylinders.
The basic idea in these patents is to create a single, laminated and proportional ink film by pressing together two cylinders, one of them metal and the other covered by a flexible material, so as to ink the offset plate directly. This procedure allows neither for zone-by-zone control of the film thickness nor for the possibility of partitioning off inks of different colors. This system is perhaps sufficient for use in offset work to handle one color or kind of ink per printing, where in theory each point on the plate takes the same quantity of ink, since nearly all offset presses in use are equipped with inking devices with zone-by-zone control screws similar to those used in typography.
In typographic printing, especially in the case of the platen press where pressure control is rather delicate depending on the lubrication of the printing parts and is closely correlated with the inking, it is essential to control the thickness of the ink film zone by zone.
Depending on their kind or color, inks have different densities and rheologies, which require different thicknesses in order to obtain a given inking.
To devise an offset or typographic inking mechanism able to handle several inks at once, where each film must be inked individually, it is necessary to take the above factor into account and provide the capacity for adjustment in narrow zones, this so as to make it possible for the printer to use narrow "ribbons" of ink.